Low pass filter incorporating coupled inductors to enhance stop band attenuation

ABSTRACT

The present invention relates to a low pass filter incorporating coupled inductors to enhance stop band attenuation. In one embodiment, the coupled inductors are provided along with various capacitors to provide for superior performance within a smaller surface area of a semiconductor or ceramic integrated device. In a further specific embodiment, the capacitors are formed on an integrated device within an area on which entirely intertwined inductors are formed. In another embodiment, at least one further pair of coupled inductors is included to create additional frequency attenuation notches, as well as a wide stop-band.

FIELD OF THE INVENTION

The present invention relates to a low pass filter incorporating coupledinductors to enhance stop band attenuation.

BACKGROUND OF THE INVENTION

Filter circuits that provide stop band attenuation are well-known. Onsuch type of circuit is a notch circuit, that filters frequencies from acertain portion of the frequency spectrum (corresponding to the notch),and allows other frequencies to pass. FIGS. 1( a) 1-2 and 1(b) 1-2illustrate a conventional prior art C-R-C and C-L-C low pass filteringcircuits, respectively, as well as their respective performancecharacteristics. As shown, they provide a stop band at about 800 MHz to6 GHz, which covers the radio frequency range used for wirelesscommunication. A steep roll off from pass band to stop band frequency ishighly desired, especially when high speed signals are incorporated inthese devices.

The present invention a circuit that provides desired stop-bandperformance in smaller areas, as well as less attenuation of the lowerfrequencies that are desired.

SUMMARY OF THE INVENTION

The present invention relates to a low pass filter incorporating coupledinductors to enhance stop band attenuation.

In one embodiment, the coupled inductors are provided along with variouscapacitors to provide for superior performance within a smaller surfacearea of a semiconductor or ceramic integrated device.

In a further specific embodiment, the capacitors are formed on anintegrated device within an area on which entirely intertwined inductorsare formed.

In another embodiment, at least one further pair of coupled inductors isincluded to create additional frequency attenuation notches, as well asa wide stop-band.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIGS. 1( a) 1-1 and 1(b) 1-1 illustrate a conventional prior artcircuit, and FIGS. 1( a)-2 and 1(b)-2 provide the frequency domainfilter performance.

FIG. 2( a) illustrates an embodiment of a coupled inductor notch circuitaccording to the present invention;

FIG. 2(B)1-2 illustrates a layout of the coupled inductor circuitaccording to FIG. 2( a) of the present invention.

FIG. 2( c) illustrates performance characteristics at different couplingratios, respectively, according to the present invention;

FIG. 2( d) illustrates a performance comparison between an embodiment ofthe present invention and a conventional C-L-C filter.

FIG. 2( e) illustrates performance characteristics at a different notchfrequency using the same circuit topology as in FIG. 2( a), according tothe present invention;

FIGS. 3( a)-(c) illustrates a circuit that uses inductors that areuncoupled, a performance curve for the circuit, and a physical layout ofthe circuit.

FIGS. 4( a)-(b) illustrate another embodiment of a coupled notch circuitaccording tot the present invention that has multiple notches and theperformance characteristics associated therewith.

FIG. 5( a) illustrates an embodiment of a coupled inductor notch circuit500 according to the present invention, and FIG. 5( b) the performancecurve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is preferably implemented on a single integratedcircuit, as is described herein. The invention is used as a filter inorder filter various different frequencies, including noise as well asupper harmonics of clock frequencies, particularly in frequency bands ofinterest such as 850 MHz. Wide passband is important for signalintegrity as it is preferred to have as many harmonics as possible topass through to preserve the waveform. In order to filter out anyun-wanted signal, a wide stopband is important.

What is the frequency range of un-wanted signal highly relies onapplication. In cell phone applications, for instance, other than theaudio filter a stopband from 0.8 GHz to 6 GHz is desired, whichcorresponding to mobile frequency range (GSM and CDMA 0.8 GHz-0.9 GHz,1.8 GHz-2 GHz, Bluetooth 2.4 GHz-2.5 GHz, wireless LAN 2.4 GHz-2.5 GHz,5.15 GHz-5.350 GHz, 5.725 GHz-5.825 GHz). The WiMax band of 2-11 GHz isalso of interest.

In a typical application, the data that is being preserved is thatdigital data with a data rate which increases. Typically it is up to 70Mbit/s—and the 5th harmonic frequency of such a data rate is 350 MHz,which signal needs to pass through.

FIG. 2( a) illustrates an embodiment of a coupled inductor notch circuit200 according to the present invention. As shown, the circuit includesan input 210, an output 220, and a ground 230, each of which, as shown,have associated therewith a parasitic resistance and a parasiticinductance. Coupled inductors 240 and 250, each having a parasiticresistance associated therewith as shown, are connected between nodes 1,2 and 3 as shown, with inductor 240 being in series with resistor 260between the input 210 and the output 220, and inductor 250 being inseries with capacitor 272 between node 2 and ground 230, with node 2being the node between inductor 250 and capacitor 272. Capacitors 270and 276 are connected between node 1 and ground and output 230 andground, respectively, as shown. An example of values for the circuitelements is provided in the Table 1 below for the FIG. 2( a) column.

TABLE 1 FIG. 2(a) FIG. 2(d) FIG. 3(a) input 210 0.3 ohm 0.3 ohm 0.3 ohmresistance input 210 1 nH 1 nH 1 nH inductance output 220 0.3 ohm 0.3ohm 0.3 ohm resistance output 220 1 nH 1 nH 1 nH inductance ground 2300.05 ohm 0.05 ohm 0.05 ohm resistance ground 230 15 pH 15 pH 15 pHinductance inductor 240 24 nH; 240 nH; 37 nH; 15 ohm 50 ohm 20 ohminductor 250 3.5 nH; 34 nH; 9.0 nH; 6 ohm 15 ohm 4 ohm resistor 260 26ohm 10 ohm capacitors 270 15 pF 145 pF 17 pF capacitor 272 2.5 pF 20 pF4 pF capacitor 274 10 pF 140 pF 6 pF

FIG. 2( b)1 illustrates a layout of the coupled inductor circuitaccording to FIG. 2( a) of the present invention. As is shown inductors240 and 250 are coupled, such that the coils of inductor 240 overlapwith the coils of inductor 250. The number and size of the coils ininductors 240 and 250 will depend on the application requirements, suchas filter pass-band and stop-band frequencies, current handlingcapability and resistance requirements. FIG. 2( b)2 shows an exampleconfiguration of a spiral inductor that can be used according to thepresent invention in a multi-layer structure.

FIG. 2( c) illustrates performance characteristics at different couplingratios, respectively, according to the present invention. As is shown,depending upon the degree of overlap (coupling coefficient K) theperformance of the circuit changes with fixed inductor size. Degree ofoverlap is most significant with respect to the surface area of thecoils that overlap, with coils that are intertwined but on separateplanes having the most overlap, and the amount of the separation betweenthe planes of the coils having an effect, but a secondary effect, on thecoupling coefficient K. The coupling coefficient K, as is known, willdepend on the degree of shared magnetic field of the two inductor coils.In a preferred embodiment, in addition to being entirely intertwined,the coils of inductors 240 and 250 are preferably positioned to thatthere the coils of one inductor are positioned in gaps between the coilsof the other inductor, thereby avoiding surface alignment of the coilsand minimizing parasitic capacitance between them. When entirelyintertwined, the typical coefficient coupling K is in the range of0.3-0.9.

FIG. 2( d) illustrates a performance comparison between an embodiment ofthe present invention and a conventional C-L-C filter. The steepattenuation and the initial notch that has greater attenuation than theconventional C-L-C circuit are noticeable and advantageouscharacteristics.

FIG. 2( e) illustrates performance characteristics at a different notchfrequency using the same circuit topology as in FIG. 2( a), according tothe present invention. The circuit values for this stop band frequencyare provided in the Table 1 above.

FIGS. 3( a)-(b) illustrates the same circuit as in FIG. 2( a), otherthan that the inductors 240 and 250 are uncoupled, and as such FIG. 3(a) shows the performance curve for the circuit, and a physical layout ofthe circuit is shown in FIG. 3( b). The circuit values for thiscomparison circuit are provided at Table 1 above as well. This circuitoccupies a significantly larger area, as there is a non-overlappinginductor layout, and there is also a larger self-inductance value foreach of the inductors 240 and 250.

FIGS. 4( a)-(b) illustrate another embodiment of a coupled notch circuitaccording to the present invention that has multiple notches and theperformance characteristics associated therewith. Circuit elements areidentified in FIG. 4( a), with an input 410, an output 420, and a ground430, each of which, as shown, have associated therewith a parasiticresistance and a parasitic inductance. One of the inductors 440 a and450 a within the coupled inductors pairs 440 a/b and 450 a/b, eachhaving a parasitic resistance associated therewith, are connectedbetween the input 410 and the output 420, and connected together at Node1. The other inductor 440 b is connected between node 1 and the ground,in series with capacitors 470; the inductor 450 b is connected betweenoutput and the ground, in series with capacitors 474; while capacitor476 is directly connected between Node 1 and ground 430. An example ofvalues for the circuit elements is provided in the Table 2.

TABLE 2 FIG. 4(a) input 410 0.3 ohm resistance input 410 1 nH inductanceoutput 420 0.3 ohm resistance output 420 1 nH inductance ground 430 0.05ohm resistance ground 430 15 pH inductance inductor 440a 20 nH; 10 ohminductor 440b 8 nH; 3 ohm inductor 450a 50 nH; 30 ohm inductor 450b 1nH; 0.5 ohm capacitor 270 2 pF capacitor 272 10 pF capacitor 274 1 pF

In this embodiment, which is also referred to as a 5-pole circuit,rather than the 3 pole circuit of FIG. 2( a), different filteringcharacteristics are achieved. In addition to more notches tosubstantially attenuate the frequencies corresponding to the notches, aswell as creates a wide stop-band corresponding to the line 490 thatshows attenuation of at least the corresponding predetermined dB amountas shown by the use of additional notches.

FIG. 5( a) illustrates an embodiment of a coupled inductor notch circuit500 according to the present invention, and FIG. 5( b) the performancecurve. As shown, the modification is an a L-C filter (instead of C-L-C)using the same concept of coupled inductors to enhance stopbandperformance. Although the performance of this circuit is not as good asC-L-C filter of FIG. 2( a) with notch, it can be used within the contextand scope of the present invention. As shown, the circuit includes aninput 510, an output 520, and a ground 530, each of which, as shown,have associated therewith a parasitic resistance and a parasiticinductance. Coupled inductors 540 and 550, each having a parasiticresistance associated therewith as shown, with inductor 540 being inseries with resistor 560 between the input 510 and the output 520, andinductor 550 being in series with capacitor 572. Capacitors 576 is alsoconnected between output 520 and ground, respectively, as shown. Anexample of values for the circuit elements is provided in the Figure.

The present invention allows for significant space savings in discretesemiconductor filter circuits, which may or may not also include ESDprotection, as well as in circuits formed on multi-layer technologies,which include not only semiconductor technologies, but also ceramictechnologies and others. The inductor, in addition to being formed in aplanar manner as shown, can also be spiral or other configurations.

Although the present invention has been particularly described withreference to embodiments thereof, it should be readily apparent to thoseof ordinary skill in the art that various changes, modifications andsubstitutes are intended within the form and details thereof, withoutdeparting from the spirit and scope of the invention. Accordingly, itwill be appreciated that in numerous instances some features of theinvention will be employed without a corresponding use of otherfeatures. Further, those skilled in the art will understand thatvariations can be made in the number and arrangement of componentsillustrated in the above figures. It is intended that the scope of theappended claims include such changes and modifications.

1. A filter circuit disposed between an input line, an output line, and a ground line, each of the input line, the output line and the ground line each having a parasitic resistance and a parasitic inductance associated therewith, the filter circuit comprising: a first inductor having first coils disposed in series with a first resistor between the input line and the output line, wherein a first node is created between the first inductor and the first resistor; a second inductor having second coils and a first capacitor disposed in series between the first node and the ground line, wherein the second coils of the second inductor at least partially overlap with the first coils of the first inductor, thereby coupling the first and second inductors and reducing a surface area occupied by the first and the second inductors; and a second capacitor disposed between the output line and the ground line; wherein the filter circuit has a predetermined stop-band, a predetermined pass-band cutoff frequency, and wherein the first and second inductors and the first and second capacitors steepen a slope of the roll-off and provide a notch at a stop-band frequency to obtain substantial attenuation beyond the cutoff frequency and minimum attenuation below the cutoff frequency.
 2. The filter circuit according to claim 1 wherein the predetermined stop-band is provided between 800-950 MHz.
 3. The filter circuit according to claim 1 wherein the predetermined stop-band is provided between 1.8-1.9 GHz.
 4. The filter circuit according to claim 1 wherein the overlap between the first and the second inductors results in the first and the second inductors being entirely intertwined.
 5. The filter circuit according to claim 1, wherein each of the first and second capacitors, the first and second inductors, and connections therebetween are formed on an integrated device that has multiple layers.
 6. The filter circuit according to claim 5 wherein the overlap between the first and the second inductors results in the first and the second inductors being entirely intertwined.
 7. The filter circuit according to claim 6 wherein the first and second inductors are further formed so that the first inductor coils are positioned in gaps between the second inductor coils, thereby avoiding surface alignment of the first and second inductor coils and minimizing parasitic capacitance between the first and second inductors.
 8. The filter circuit according to claim 7 wherein the first and second inductors are planar and are formed on different layers of the integrated device.
 9. The filter circuit according to claim 8 wherein the first and the second capacitors are each formed within a surface area of the first and the second inductors.
 10. The filter circuit according to claim 5 wherein the integrated device is a semiconductor device.
 11. The filter circuit according to claim 10 the predetermined stop-band is provided between at least one of 800-950 MHz and 1.8-1.9 GHz.
 12. The filter circuit according to claim 5 wherein the integrated device is a ceramic device.
 13. The filter circuit according to claim 12 wherein the predetermined stop-band is provided between at least one of 800-950 MHz and 1.8-1.9 GHz.
 14. The filter circuit according to claim 1 further including: a third inductor having third coils disposed between the input line and the first inductor; and a fourth inductor having fourth coils and having one end disposed between a node formed by the first inductor and the third inductor, and another end connected to the first capacitor, wherein the third coils of the third inductor at least partially overlap with the fourth coils of the fourth inductor, thereby coupling the third and fourth inductors, reducing a surface area occupied by the third and the fourth inductors and creating a further attenuation frequency notch.
 15. The filter circuit according to claim 14 wherein the third and fourth inductors, along with the first and second inductors and the first and the second capacitors create a wide stop-band corresponding to a predetermined attenuation amount.
 16. The filter circuit according to claim 14, wherein each of the first and second capacitors, the first, second, third and fourth inductors, and connections therebetween are formed on an integrated device that has multiple layers.
 17. The filter circuit according to claim 16 wherein the overlap between the first and the second inductors results in the first and the second inductors being entirely intertwined and the overlap between the third and the fourth inductors results in the third and the fourth inductors being entirely intertwined.
 18. The filter circuit according to claim 17 wherein: the first and second inductors are further formed so that the first inductor coils are positioned in gaps between the second inductor coils, thereby avoiding surface alignment of the first and second inductor coils and minimizing parasitic capacitance between the first and second inductors; and the third and fourth inductors are further formed so that the third inductor coils are positioned in other gaps between the fourth inductor coils, thereby avoiding surface alignment of the third and fourth inductor coils and minimizing parasitic capacitance between the third and fourth inductors.
 19. The filter circuit according to claim 18 wherein the first and second inductors are planar and are formed on different layers of the integrated device; and wherein the third and fourth inductors are planar and are formed on different layers of the integrated device.
 20. The filter circuit according to claim 18 further including a third capacitor disposed between the input line and the ground line.
 21. The filter circuit according to claim 20, wherein each of the first, second and third capacitors, the first, second, third and fourth inductors, and connections therebetween are formed on an integrated device that has multiple layers.
 22. The filter circuit according to claim 20, wherein each of the first, second and third capacitors, the first, second, third and fourth inductors, and connections therebetween are formed on an integrated device that has multiple layers.
 23. The filter circuit according to claim 17 wherein the first and second capacitors are each formed within a surface area of the first, second, third and fourth inductors.
 24. The filter circuit according to claim 16 wherein the integrated device is a semiconductor device.
 25. The filter circuit according to claim 24 wherein the predetermined stop-band is provided between at least one of 800-950 MHz and 1.8-1.9 GHz.
 26. The filter circuit according to claim 24 wherein the predetermined stop-band is provided between at least one of 800-950 MHz and 1.8-1.9 GHz.
 27. The filter circuit according to claim 16 wherein the integrated device is a ceramic device.
 28. The filter circuit according to claim 1 further including a third capacitor disposed between the input line and the ground line. 